The long-term goal of these studies is definition of the role of cyclic nucleotides as regulators of mammalian gastric secretion with particular emphasis on the source and activity of gastric cyclic nucleotides, and the "second messenger system" of cyclic nucleotides as mediators of gastric acid secretion in response to gastrointestinal hormones, other hormones, cholinergic release, adrenergic blockade, fasting, refeeding, H2 receptor-antagonism, hyperchlorhydria, achlorhydria and phosphodiesterase inhibition by theophylline. Gastric juice was collected after constant infusion with histamine, pentagastrin, bethanechol or theophylline in 4 conscious or anesthetized trained mongrel dogs (1 with an antral pouch and the other 3 with a Heidenhain pouch). In acute studies, the intact stomach was evaluated by mucosal biopsy in 33 anesthetized mongrel dogs who were fasted overnight and anesthetized with intravenous sodium pentobarbital, 30 mg/kg. Antral and fundic gastric mucosal cyclic nucleotide activity did not differ from control levels after stimulation with either histamine, pentagastrin, or bethanechol. There were no significant differences between antral or fundic mucosal cyclic nucleotide concentrations in the basal state or in the stimulated state. Concomitant administration of histamine, pentagastrin and bethanechol with theophylline did not potentiate the increment change in response of any of these agents compared to theophylline alone. Theophylline did not affect gastric acid output, volume or cyclic nucleotide concentrations in gastric juice, but significantly increased cAMP and cGMP in antral and fundic mucosa. Our results in canine gastric secretion provide evidence against the hypothesis that cAMP or cGMP are mediators for the initiation of acid secretion in mammalian stomach by histamine, pentagastrin or bethanechol.